The present invention relates to bearingless machine drives.
Conventional magnetic bearing motor drive systems include a five axis magnetic bearing system and a motor drive. In these embodiments, the motor drive, two radial bearings, and an axial bearing are situated on a common shaft with the two radial bearings being positioned on opposite sides of the motor drive. A separate inverter is needed for each bearing and the motor drive.
Bearingless motors combine the function of an electric motor to generate torque with the function of a magnetic bearing to generate radial forces. The torque and radial forces are generated by adding a control winding to the motor with a pole number which differs from the main motor winding (the drive winding) by two poles.
The interaction of the two air-gap fields differing by one pole pair results in a net radial force acting on the rotor. Controllable radial forces can then be produced by appropriately controlling the current of the control winding. Such control techniques are discussed, for example, in Schob, U.S. Pat. No. 5,708,346, and Fukao et al., EP Patent Application No. 739 078.
A typical five-axis bearingless motor system performing the same function as a magnetic bearing system includes two motors with the rotors joined together on a common shaft. Each motor generates controllable radial forces and thus permits stable control of the rotor. Because each motor can only produce radial forces in two axes, two motors are required to achieve stable levitation. End windings of the stators and end rings of the rotors often require a space which creates a large separation between the two motors.